Lady Gaga 'stopped dead' by Bradley Cooper's singing talents

Movie veteran Sam Elliott felt like he'd become like a brother to Cooper after filming wrapped.

Lady Gaga was completely awestruck the first time she heard Bradley Cooper sing.

The pop superstar was recruited by Bradley to feature opposite him in his directorial debut, a remake of hit movie musical A Star Is Born, about a troubled seasoned musician who discovers and falls in love with a struggling wannabe singer.

The film is a modern revamp of the classic versions starring Judy Garland and Barbra Streisand, who tackled the female lead in 1954 and 1976, respectively, and Gaga was so determined to ensure the performance portions of the new A Star Is Born were authentic, she insisted the songs were belted out live onset.

To achieve the feat, Cooper underwent 18 months of vocal training, and Gaga reveals it really paid off, because she was blown away when the actor showed her what she'd be working with.

"He began to sing, and I stopped dead in my tracks, and I said, 'Oh my gosh, Bradley, your voice!'" the six-time Grammy Award winner recalled during a cast interview on breakfast show Good Morning America. "He can sing!"

The two stars ended up helping each other on set, with Cooper giving the musician a little acting advice, and Gaga coaching the Oscar nominee on his vocals, whenever needed.

"What he taught me was know your lines, but really know the story that you're telling, what you're trying to say," she recalled.

Praising his talents as a filmmaker, Gaga added, "Bradley really has been an incredible leader, creating a beautiful family."

The bond formed between the actor-turned-director and his castmates was also unlike any other their veteran co-star Sam Elliott had experienced throughout his decades-long career.

"I've never been anywhere near someone like (Gaga) that's as huge as she is on a worldwide level, and somebody that's as talented as she is," he gushed. "And I've never met anyone like Bradley since I've been in the business, and come out the other side really feeling like his brother. I'm just really fortunate to be sitting here, and that's the truth of it."

A Star Is Born, which has already received rave reviews, opens in theatres in October (18).

It marks Gaga's first leading role in a movie, after previously scoring small appearance in films like Machete Kills and Sin City: A Dame to Kill For, as well as featuring in TV series American Horror Story: Hotel, which earned her the Golden Globe Award for Best Actress - Mini-Series or Television Film in 2016.